In the previous study (1997), we demonstrated that ADP-ribosylation factor (Arf) is present in the cytosolic fraction of rat parotid acinar cells and Arf translocates to the secretory granules in a GTp-dependent manner. So far, six mammalian Arf genes (Arf1-Arf6) have been reported. In bovine adrenal chromaffin cells, Arf6 is associated to the secretory granules. Now we have succeeded in demonstrating the following two points to elucidate the role of Arf proteins on the secretory granules. These results were published in Archives of Biochemistry and Biophysics 357, 147-154 (1998).1. Identification of the Arf isotype as Arfi1that translocates to the secretory granulesWe identified the Arf isotype that translocates to the secretory granules by using antibodiesspecific for Arf1, Arf3, Arf5, and Arf6. In contrast to the Arf6 localization to the secretory granules in bovine adrenal chromaffin cells, Arf6 was not detected in the granule membrane fraction of rat parotid acinar cells.2. Demonstration of the Arfi translocation to the secretory granules by immunoelectron microscopyTo confirm directly that Arf1 is translocated to the intact secretory granules, the secretorygranules were isolated after the translocation experiment and used for the immunoelectron microscopy with anti-Arf antibody. Gold particles were detected around the surface of the mature secretory granules, about 1 mum in diameter.